Grid Floor Has Randomly Disappeared
In the properties panel, under Display / Grid floor, is a Scale parameter. It seems that in your first picture, you set it to 10 (look top left of the 3DView, it says "Meters x10"). That means each square of the grid is 10m ; but your Plane is just a few cm.

Just set the grid Scale to a more appropriate value (0.01 seems good : 1cm for each square of the grid).